The reversible wetting transition between superhydrophilicity and superhydrophobicity of tremella-like CuxO@CuxS nanosheets prepared by one-step anodization and the application of on-demand oil/water separation

https://doi.org/10.1016/j.jallcom.2021.161793Get rights and content

Highlights

  • Large scale tremella-like CuxO@CuxS nanosheets are prepared by anodizing Cu mesh.

  • Stable reversible wetting transition is induced by alternative heating and storage.

  • The reversible wetting transition mechanism is analyzed in detail by XPS.

  • It has excellent mechanical stability and corrosion resistance on Cu mesh.

  • It has successfully applied in high efficient on-demand oil/water separation.

Abstract

A simple one-step anodizing process is developed for the large-scale preparation of the tremella-like CuxO@CuxS nanosheet films on Cu mesh, and the prepared organic-free CuxO@CuxS nanosheet films on Cu mesh exhibit excellent mechanical stability and corrosion resistance. The fresh organic-free tremella-like CuxO@CuxS nanosheets have the superhydrophilicity and underwater superoleophobicity. But after storage in atmosphere for about 6 days under room temperature, the wettability is gradually transformed into superhydrophobicity and superlipophilicity. Importantly, the wettability can turn back again to the superhydrophilicity and underwater superoleophobicity after the annealing process at 200 °C in air environment. So the stable reversible wetting transition is realized, then the transition mechanism has been investigated by the detailed X-ray photoelectron spectroscopy (XPS) characterization, and is mainly attributed to the production of hydroxide radicals and adsorption of C-C/C-H species on the CuxO@CuxS nanosheet surface. Lastly, the prepared tremella-like CuxO@CuxS nanosheets on copper mesh have been successfully applied for on-demand oil/water separation with high efficiency.

Introduction

The oily sewage discharged from chemical industry and daily life has caused serious water pollution and marine pollution [1], [2]. Therefore, the development of techniques and methods for the large-scale oil-water separation becomes a very urgent task [3]. Especially, as the application environment of oil-water separation becomes more and more complex, the on-demand oil/water separation gradually grows a critical focus to meet the complex environment changes. Inspired by the nature, the porous solid with the entirely opposite wettability towards water and oil gives a simple and effective way for the oil/water separation [4], [5], [6]. The meshy superhydrophobic and superoleophilic solid is mainly used to filtrate oil from heavy oil/water mixture, named as “oil removal” mode. The superoleophobicity and superhydrophilicity is suitable to remove water from light oil/water mixture, named as “water removal” mode [7], [8]. Over the past two decades, a large amount of special superwetting materials with entirely opposite affinities to water and oil has been prepared for the oil/water separation [9], [10]. But most materials are suitable for only one certain separation mode. Recently, the stimuli-responsive superwetting materials are continually developed. Especially when they have reversible wetting responses, they can be applied for the on-demand oil/water separation [11], [12]. According to the oil density and environment changes, the wettability of filtration materials can be adjusted to meet oil/water separation. But the simple, large scale preparation of the stable, cost-effective on-demand oil/water separation materials remains a challenge.

Copper mesh has excellent durability, flexibility and air permeability, is ideal and favored base material for oil/water separation [13], [14]. But the copper mesh itself do not have the oil/water separation function, so the surface treatment becomes necessary to achieve the completely opposite wettability towards oil and water. Generally, surface treatment includes the introduction of surface micro/nanostructure and surface modification of chemical materials. On the one hand, an organism with a hydrophobic or hydrophilic group is directly modified on solid surface to fulfill the superwetting surface. But it is difficult to realize reversible switch between superhydrophilicity and superhydrophobicity. On the other hand, the surface is introduced plenty of active sites to adsorb hydrophobic or hydrophilic groups, and further modulate the surface wettability. And the adsorbed hydrophobic or hydrophilic groups are usually desorbed by some post-treatments, such as heating, light irradiation, and so on. Obviously, this route provides an ideal way to realize the reversible switch between superhydrophilicity and superhydrophobicity. Extensive researches have been carried out to verify the effectiveness of the way [15], [16]. Copper sulfide (CuS) is an important transition metal chalcogenide semiconductors with narrow band gap of about 2 eV, and can be easily introduced sulfur vacancy defects on the surface to form cuprous ions, which serves as the active sites to adsorb the hydrophobic or hydrophilic groups in air. Thus, CuS is very suitable for the surface modification on the copper mesh to achieve the stimuli-responsive superwetting state [17]. Up to now, various CuS micro/nanostructures had been prepared [18], [19], and it is found that annealing, storage in air, light irradiation, etc., can effectively adjust the CuS surface wettability [20], [21], and even achieve reversible wetting transition. However, the reversible wetting transition mechanism is still a disputed focus [22], and the wetting transition still needs too long time, even 14 days. Furthermore, the preparation methods of CuS usually are microemulsion, sol-gel, dipping coating, solvothermal method, and so on [23], [24], [25]. But the prepared CuS nanostructures on copper mesh have poor controllability and poor adhesion to the substrate. Lastly, in order to achieve the supperwetting surface, some organic materials are usually modified on these structured surfaces [26]. Obviously, these adverse factors greatly restrain the application of copper mesh modified with CuS in on-demand oil/water separation.

Thus, in this work, a simple one-step anodizing process for the large scale preparation of robust tremella-like CuxO@CuxS nanosheets on copper mesh is reported. The freshly prepared tremella-like CuxO@CuxS nanosheets without surface modification of organic materials has the superhydrophilicity and underwater superoleophobicity. After a short period of storage in atmosphere for only 6 days under room temperature, the wettability is gradually transformed into superhydrophobicity and superlipophilicity. Importantly, the wettability can turn back the superhydrophilicity and underwater superoleophobicity after the annealing process at 200 °C in air environment. So the stable reversible wetting transition is realized and the mechanism of revisable wetting transition is also detailedly investigated. Lastly, the prepared tremella-like CuxO@CuxS nanosheets on copper mesh have been successfully applied for on-demand oil/water separation of high separation efficiency.

Section snippets

The preparation of the tremella-like CuxO@CuxS nanosheets

The anodization process was used to prepare samples [27]. As shown in Fig. 1, the tailored Cu meshes (99.999%, 10 mm × 20 mm × 0.2 mm) were firstly cleaned by ultrasound in acetone solution, then were transferred into the electrolytes including nitric acid and ultrapure water with the volume ratio of 1/3 to be chemical polished, and further post-processed and cleaned with ethanol and deionized water. Secondly, the polished Cu meshes were put into the 0.5 mol/L Na2S solution to perform the

Morphologies and composition

Fig. 2(a) shows the top-view SEM image of the anodized film for 180 s, and the inset shows the corresponding cross section. The tremella-like nanosheets with number density of 13.50 µm−2 stands vertically and uniformly on the surface of each copper wire woven into copper mesh. The average thickness of nanosheet is about 122.1 nm, average interval between nanosheets is 0.224 µm around, and average nanosheet height (i. e. film thickness) is about 866.7 nm. As the anodizing time increases

Conclusions

In a word, a facile one-step anodizing process to prepare the tremella-like CuxO@CuxS nanosheets on copper mesh is developed. The prepared CuxO@CuxS nanosheets have corrosion resistance and abrasion resistance. The fresh and annealed CuxO@CuxS nanosheets on copper mesh have the superhydrophilicity and underwater superoleophobicity. After storage in atmosphere for only 6 days under room temperature, the wettability is gradually transformed into superhydrophobicity and super-oleophilicity, but

CRediT authorship contribution statement

Shu Liu: Investigation, Writing – original draft. Jian Wang: Conceptualization, Methodology, Investigation, Writing – review & editing. Xinyu Pei: Investigation. Xianggang Dai: Investigation. Yan Li: Writing – review & editing. Jianbiao Chen: Writing – review & editing. Chengwei Wang: Writing – review & editing.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

The Authors are grateful to the support of the National Natural Science Foundation of China (Grant No. 11464041, 11864035, 12064039) and the Natural Science Foundation of Gansu Province of China (Grant No. 20JR5RA537).

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